European Journal of Cardiovascular Medicine

The flexor digitorum superficialis (FDS) tendon plays a crucial role in hand function, particularly facilitating finger flexion at the proximal interphalangeal joints. Anatomical variations of the FDS tendons, though often asymptomatic, have substantial surgical relevance, especially during tendon repair, graft harvesting, and reconstructive procedures [1]. A thorough understanding of these variations is paramount for hand surgeons to avoid iatrogenic injuries and to devise effective surgical strategies.

Several studies have documented congenital absence, duplication, fusion, and anomalous insertion patterns of FDS tendons, with significant variability observed across different populations and ethnic groups [2,3]. For example, absence of the FDS to the little finger has been reported with a prevalence ranging from 10% to 21% [4], with important implications for tendon transfer procedures and flexor tendon repairs.

Furthermore, anatomical knowledge of FDS variations is crucial during interventions such as flexor tendon grafting, opponensplasty, and even during diagnostic manoeuvres like the FDS integrity tests [5,6]. Failure to recognize such variations may result in misinterpretation of clinical assessments or inadvertent injury during surgical exploration.

With the increasing frequency of hand surgeries, including trauma reconstructions and corrections of congenital anomalies, the need for detailed preoperative anatomical knowledge has become more pronounced [7]. The flexor apparatus, owing to its intricate arrangement and inherent variability, demands meticulous attention during operative procedures to minimize functional impairment [8].

Despite numerous anatomical studies, there remains a paucity of region-specific data concerning FDS tendon variations in the Indian population, particularly in eastern India [9]. This study was conducted at Patna Medical College and Hospital (PMCH) to document the anatomical variations of the FDS tendons in a cohort of 100 patients and assess their clinical and surgical relevance.

Understanding the spectrum of FDS tendon variations can enhance surgical planning, optimize clinical outcomes, and reduce intraoperative surprises, particularly in resource-limited settings where preoperative imaging may not always be feasible [10].

This prospective observational study was conducted at the Department of Anatomy and Hand Surgery Unit, Patna Medical College and Hospital (PMCH), Patna, Bihar, from January 2024 to December 2024. A total of 100 patients undergoing elective hand surgeries or cadaveric hand dissections were included. Ethical approval was obtained from the Institutional Review Board (IRB) prior to study initiation.

Eligibility criteria included adults aged between 18 and 70 years, either undergoing hand surgeries requiring tendon exposure or cadaveric donors with well-preserved hand anatomy. Exclusion criteria were patients with severe hand trauma resulting in tendon loss, history of tendon grafting or repair surgeries, or congenital syndromes involving multiple soft tissue anomalies, to avoid confounding anatomical alterations.

During surgical exposure or anatomical dissection, a detailed assessment of the flexor digitorum superficialis (FDS) tendons was performed. Each tendon destined for the index, middle, ring, and little fingers was carefully identified. Variations such as congenital absence, fusion of adjacent tendons, abnormal insertions, or duplications were meticulously recorded. Photographic documentation was supplemented with schematic drawings for all anomalous findings to ensure comprehensive recording.

For each case, laterality (right vs. left hand) and digit-specific variations were documented. Particular attention was given to the little finger, given its known predisposition to FDS anomalies.

The primary outcome measure was the prevalence of anatomical variations in FDS tendons. Secondary outcomes included the distribution of variations by hand side (right vs. left) and by individual digit involvement.

Data were collected using standardized proformas and subsequently tabulated for statistical analysis. Descriptive statistics were used to summarize the findings. Categorical variables were expressed as frequencies and percentages. Comparisons between right and left hands were performed using the Chi-square test. A p-value less than 0.05 was considered statistically significant. Statistical analysis was conducted using SPSS version 26.0 (IBM Corp., Armonk, NY).

A schematic study flowchart is provided in Figure 3.

Figure 3: Study flowchart summarizing participant selection, flexor digitorum superficialis (FDS) tendon evaluation, identification of anatomical variations, and subtype classification.

Study Population

A total of 100 participants were included in the study, comprising 82 males (82%) and 18 females (18%). The mean age of the participants was 39.6 ± 12.4 years, ranging from 18 to 68 years.

Out of the 100 hands examined:

• 53 hands (53%) were right hands and
• 47 hands (47%) were left hands.

No significant difference was noted between right- and left-hand inclusion rates (Chi-square test, p = 0.412).

The demographic and laterality characteristics of the study population are summarized in Table 1.

Table 1: Baseline Characteristics of Study Participants

Prevalence of FDS Tendon Variations

Anatomical variations of the flexor digitorum superficialis (FDS) tendons were observed in 34 out of 100 hands examined, yielding an overall variation prevalence of 34%.

The specific types of anatomical variations documented were:

• Congenital absence of FDS to one or more digits: 21 cases (21%)
• Fusion of adjacent FDS tendons: 7 cases (7%)
• Duplication of FDS tendons: 4 cases (4%)
• Anomalous insertion patterns: 2 cases (2%)

The most common variation identified was the absence of the FDS tendon to the little finger, accounting for 17 cases (17%) out of the total study population.

A detailed distribution of the types and frequencies of FDS tendon variations is presented in Table 2.

Table 2: Types and Frequencies of FDS Tendon Variations (n = 100 hands)

Digit-Specific Distribution

Among the 34 hands demonstrating anatomical variations, the little finger was the most frequently involved digit, accounting for 23 cases (67.6%). Variations involving the ring finger were observed in 6 cases (17.6%), followed by the middle finger in 4 cases (11.8%). Variations affecting the index finger were rare, noted in only 1 case (2.9%).

The absence of the FDS tendon to the little finger was the single most prevalent finding, observed in 17 hands (50% of all variations). Fusion and duplication anomalies were distributed more evenly across the little, ring, and middle fingers, whereas anomalous insertions were confined to the ring and little fingers.

A detailed breakdown of digit-specific variation patterns is presented in Table 3.

Table 3: Digit-Specific Distribution of FDS Tendon Variations (n = 34 hands with variations)

Laterality Analysis

Among the 34 hands exhibiting FDS tendon variations:

• 19 hands (55.9%) were right hands
• 15 hands (44.1%) were left hands

Although variations were slightly more common on the right side, the difference was not statistically significant (Chi-square test, χ² = 0.529, p = 0.467).

No particular type of variation (e.g., congenital absence, fusion, duplication) showed significant side predilection.

The distribution of anatomical variations according to hand side is summarized in Table 4.

Table 4: Laterality Distribution of FDS Tendon Variations (n = 34 hands with variations)

(Chi-square test: χ² = 0.529; p = 0.467)

Detailed Subtypes of Variations

Among the anatomical variations observed, the most common subtype was the congenital absence of the FDS tendon to the little finger, noted in 17 out of 34 hands with variations (50%).

Fusion of adjacent FDS tendons was identified in 7 hands (20.6%), predominantly involving the tendons to the ring and little fingers.

Duplication of the FDS tendon was seen in 4 hands (11.8%), primarily affecting the middle and ring fingers.

Anomalous insertion patterns were observed in 2 hands (5.9%), characterized by atypical tendinous slips merging into the lumbrical muscles or lateral bands rather than inserting into the middle phalanx.

The detailed distribution of these subtypes is summarized in Table 5.

Table 5: Subtypes of FDS Tendon Variations (n = 34 hands with variations)

Figure1. Figure 1: Subtype Distribution of FDS Tendon Variation

Figure 1: Subtype distribution of flexor digitorum superficialis (FDS) tendon variations among 34 hands. The most common anomaly was congenital absence of the FDS to the little finger (50%), followed by fusion (20.6%), duplication (11.8%), anomalous insertion (5.9%), and other minor variations (11.8%).

Association of Sex with FDS Tendon Variations

Among the 82 male participants, 30 males (36.6%) exhibited anatomical variations, whereas among the 18 female participants, 4 females (22.2%) exhibited variations.

Although the prevalence of FDS variations was higher in males compared to females, the difference was not statistically significant (Chi-square test, χ² = 1.312, p = 0.252).

The odds ratio (OR) for males having an FDS variation compared to females was 2.01 (95% CI: 0.61–6.68), suggesting a non-significant but higher likelihood of variations among males.

The distribution is summarized in Table 6.

Table 6: Association Between Sex and FDS Tendon Variations (n = 100 hands)

(Chi-square test: χ² = 1.312; p = 0.252; Odds Ratio = 2.01; 95% CI: 0.61–6.68)

Age Group–Wise Prevalence of FDS Tendon Variations

When participants were stratified by age groups:

• Among participants aged <30 years (n = 28), 6 hands (21.4%) demonstrated variations.
• In the 30–50 years group (n = 52), 20 hands (38.5%) exhibited variations.
• In those aged >50 years (n = 20), 8 hands (40.0%) demonstrated variations.

Although a trend toward increased prevalence of variations with advancing age was observed, the difference across age groups was not statistically significant (Chi-square test, χ² = 3.312, p = 0.191).

The distribution is summarized in Table 7 and figure 2.

Table 7: Age Group–Wise Distribution of FDS Tendon Variations (n = 100 hands)

(Chi-square test: χ² = 3.312; p = 0.191)

Figure2: Percentage of participants with FDS tendon variations across sex (male, female) and age groups (<30, 30–50, >50 years)

Figure 2: Distribution of flexor digitorum superficialis (FDS) tendon variations by sex and age group. A higher prevalence was noted among males and participants aged over 30 years, although these differences were not statistically significant.

The flexor digitorum superficialis (FDS) tendon plays a critical role in fine motor control and grip strength, particularly through flexion at the proximal interphalangeal joints. Understanding its anatomical variations is crucial for optimizing surgical outcomes in tendon repair, grafting, and reconstructive hand procedures.

In the present study, anatomical variations of the FDS tendon were identified in 34% of examined hands, with the most common anomaly being congenital absence of the FDS to the little finger (50% of variations). Fusion of adjacent tendons (20.6%), duplication of tendons (11.8%), and anomalous insertions (5.9%) were also observed.

Our findings are consistent with those reported by Tan et al. [11], who noted that independent FDS function in the little finger was present in 51.4% of examined hands using a novel examination method, and that significant connections between FDS tendons of the little and ring fingers could mimic or mask true absence. Similarly, Tan et al. [12] emphasized that absent FDS function, especially in the little finger, could not always be presumed solely based on clinical examination, owing to the presence of FDP substitutions and cross-connections.

The prevalence rates observed in our study are comparable to those reported in other population studies. A large-scale Brazilian study by Artigos Originais et al. [13] found a prevalence of 34.53% absence of FDS function to the fifth digit in the right hand and 30.06% in the left hand, figures remarkably close to our findings. Interestingly, they also reported a statistically significant higher prevalence in females, a trend that was not statistically significant in our sample.

In cadaveric observations, Rao et al. [14] described rare cases of separate muscle bellies and tendons specifically targeting the fifth digit, an important anatomical variant that could have implications during tendon harvesting or surgical repair procedures. The surgical relevance of such variations has been further stressed by Shrewsbury and Kuczynski [15], who emphasized the importance of recognizing variant tendon paths to avoid intraoperative confusion.

Ethnic and regional variability has also been reported. Irmak et al. [16], in a Turkish population study, found the prevalence of FDS fifth digit variations to be 23.7%, suggesting a substantial inter-population variation, although methodological differences across studies must be acknowledged.

The clinical implications of these anatomical variants are significant. Failure to recognize absent or variant FDS tendons can lead to misinterpretation of preoperative diagnostic tests, incorrect surgical planning, or intraoperative surprises, particularly during procedures like tendon transfers, flexor tendon grafting, and opponensplasties. Preoperative physical examinations, supplemented by awareness of possible tendon anomalies, can help mitigate these risks.

The strengths of our study include a focused examination of the FDS variations in an Indian cohort, a relatively underreported population group, and the use of both surgical and cadaveric observations to maximize detection rates. However, limitations include the single-centre design and sample size, which may not fully capture the variability present in the broader population.

Anatomical variations of the FDS tendons are common and clinically significant. Recognition of these variations is essential for surgeons to optimize functional outcomes in hand surgery. Future studies with larger, multicentric datasets and preoperative imaging correlations may help further elucidate the clinical impacts of these anatomical variations.

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